Retroviral vectors have been used to transfer and express the cDNAs for drug metabolizing enzymes (cytochromes P450 2A6 and 4B1, FMO, and PHS-1 & PHS-2) into C3H10T1/2 and AS52 cells. C3H10T1/2 cells expressing the human cytochrome P450 2A6 were mutated and morphologically transformed by the tobacco specific nitrosamine, NNK. AS52 cells expressing 2A6 were mutated by NNK and the mutational spectra of this potential human carcinogen determined. C3H10T1/2 cells expressing rabbit cytochrome P450 4B1 were sensitive to the cytotoxic effects of ipomeanol and AF and have been shown to be mutated by ipomeanol. AS52 cells containing the rabbit lung FMO and mouse PHS-1 AND PHS-2 have recently been derived. Studies to determine if mutational spectra vary when a chemical is activated by different P450s or species dependent isoforms are in progress.